Wireless communication devices such as smartphones are being used on an increasingly global basis, and devices that can be used in multiple countries are in demand. In recent years, for example, communication systems such as LTE have begun to be introduced in many countries. However, in many cases the frequency bands used in each country are different. As a result, in recent years the number of frequency bands that a single wireless communication device must support has increased.
Wireless communication devices include circuits that include duplexers and various types of filters for each frequency band established in the communication systems used. Diplexers and semiconductor switches are provided between these circuits and an antenna. Due to the increasing sophistication of communication technologies in recent years, devices that can simultaneously communicate between these systems are in demand, and diplexers that function as the interfaces between the circuits for each communication system (frequency band) and the antenna have attracted increased attention. A diplexer is a filter circuit for multiplexing and demultiplexing signals on a plurality of frequency bands.
Wireless communication devices utilize circuit components such as directional couplers and balanced/unbalanced converters (baluns) in order to implement power control during communication, for example. Couplers extract a portion of the output power from a power amplifier (PA). Baluns convert balanced signals to unbalanced signals or convert unbalanced signals to balanced signals.
Complex circuits in which a diplexer and a balun, for example, are integrated together have already been developed and brought to market (see Patent Document 1, for example). These types of complex circuits in which several circuit components are integrated together make it possible to achieve further miniaturization of the electronic devices (such as wireless communication devices) that are equipped with such complex circuits.
The complex circuit disclosed in Patent Document 1, for example, includes a single balun and a multiplexing/demultiplexing circuit (a diplexer). The balun includes four lines, among which an unbalanced transmission line and a pair of balanced transmission lines are included. The multiplexing/demultiplexing circuit includes a low-pass filter and a high-pass filter. In the overall complex circuit, the low-pass filter and the high-pass filter of the multiplexing/demultiplexing circuit are connected in parallel to the unbalanced transmission line of the balun.